1. Field of the Invention
The present invention relates to a toner system printer for carrying out printing by using a toner such as a laser system printer or a copying machine, and more particularly to a high voltage generating circuit for generating a high voltage to be required for toner system printing.
2. Description of the Related Art
JP-A-63-87173 has disclosed the conventional art in which a switching operation is changed into an intermittent operation to prevent the breakdown of an oscillating element when an output is short-circuited in a switching power source. In this technique, a resistor for detecting a current is inserted in a current path of an emitter of a switching transistor. When a voltage of the emitter exceeds a predetermined voltage, a current of the switching transistor is controlled to be a constant current. Moreover, a comparator for detecting that a load is short-circuited is provided. When a short circuit is caused, the switching operation of the switching transistor is stopped by the comparator. Furthermore, when a charging period of a capacitor for soft start is passed after the switching operation is stopped, the switching is restarted. In other words, when the load is short-circuited, the switching transistor carries out intermittent oscillation. Therefore, the switching transistor is prevented from being broken when the load is short-circuited (which is a first conventional art).
Moreover, JP-A-9-9616 has disclosed a technique. In this technique, when the short circuit of an output is detected, constant current charging for a capacitor is started. When a terminal voltage of the capacitor exceeds a first voltage, an internal bias source is turned OFF, thereby stopping the driving operation of a switching transistor. Then, when the terminal voltage of the capacitor is raised to be a second voltage which is higher than the first voltage, the capacitor is discharged to have an initial voltage, thereby restarting the driving operation of the switching transistor. In other words, when a load is short-circuited, the switching transistor is intermittently driven. Therefore, it is possible to prevent the switching transistor from being broken when the load is short-circuited (which is a second conventional art).
In a method in which a switching element is brought into a saturation state when it is turned ON as in the switching method according to the technique described above, large output power can cause efficient conversion to be carried out. However, in the case in which slight output power is enough as in a high voltage generating circuit for a toner system printer, a conversion efficiency is deteriorated and an unstable operation is caused. For this reason, the high voltage generating circuit for a toner system printer employs such a structure that a transistor for oscillation which is connected to a primary coil is always carries out self-oscillation in a non-saturation region and an amplitude of oscillation is controlled to stabilize an output voltage. In other words, there has been employed a structure in which an output voltage is stabilized by a method different from the first conventional art and the second conventional art.
FIG. 7 shows the structure described above, that is, a conventional art of a high voltage generating circuit for stabilizing an output voltage by controlling an amplitude of oscillation of a transistor for oscillation. More specifically, a transistor for oscillation Q1 (which will be hereinafter referred to as a transistor Q1) has a collector connected to a primary coil L1 and a base connected to one of terminals of an auxiliary coil L3. Moreover, an output of an OP amplifier 3 for generating a voltage control signal based on a voltage of a secondary output is led to the other terminal of the auxiliary coil L3. For this reason, the transistor Q1 increases an amplitude of oscillation when a voltage of the voltage control signal sent from the OP amplifier 3 is raised, and reduces the amplitude of the oscillation when the voltage of the voltage control signal is dropped. On the other hand, the OP amplifier 3 reduces the voltage of the voltage control signal when the voltage of the secondary output is raised to be higher than a set value, and raises the voltage of the voltage control signal when the voltage of the secondary output is reduced to be lower than the set value. Therefore, the voltage of the secondary output is stabilized to be a voltage set based on a voltage setting signal 22 (the reference numeral 21 denotes a path for applying a reference voltage to set an operating point of the OP amplifier 3).
In the structure described above, that is, such a structure that the voltage of the secondary output is stabilized by controlling the amplitude of the oscillation, the transistor Q1 stops the oscillation even if the oscillation is not externally stopped in respect of the relationship in which a load is too increased if an output is short-circuited. On the otherhand, in the case in which the oscillation of the transistor Q1 is stopped, the voltage of the output (voltage control signal) of the OP amplifier 3 is raised so that a base current of the transistor Q1 is increased. Therefore, a collector current is increased. More specifically, when the transistor Q1 is brought into an oscillation stop state, a calorific value becomes greater than that in the oscillation state so that an element is broken more easily. For this reason, the methods according to the first and second conventional arts (the structure in which a switching element is brought into a saturation state during turnxe2x80x94on) cannot be used as a method for carrying out protection when an output is short-circuited. Therefore, another method is used to protect the transistor Q1.
More specifically, when the output is short-circuited, the voltage of the voltage control signal sent from the OP amplifier 3 is raised. Therefore, there has been employed such a structure that a resistor R21 is inserted between the output terminal of the OP amplifier 3 and the other terminal of the auxiliary coil L3 and a clamp circuit comprising diodes D5 and D6 is connected between the resistor R21 and the auxiliary coil L3. Accordingly, the voltage of the other terminal of the auxiliary coil L3 is controlled to be approximately 1.3 V also when the output voltage of the OP amplifier 3 is raised. As a result, an increase in the base current of the transistor Q1 is restricted and an increase in the collector current is suppressed. Consequently, the transistor Q1 can be protected from breakdown (which is a third conventional art).
Moreover, FIG. 8 shows a structure in which a different method from the third conventional art is employed for protecting the transistor Q1. More specifically, a portion comprising the resistor R21 and the diodes D5 and D6 in FIG. 7 is replaced by a resistor R22 and a Zener diode D7 (other portions are identical to those of the structure shown in FIG. 7). In such a structure, accordingly, the voltage of the other terminal of the auxiliary coil L3 is shifted to the lower side by a Zener voltage of the Zener diode D7 with respect to the output voltage of the OP amplifier 3. Therefore, the voltage of the other terminal of the auxiliary coil L3 is controlled to be low also when the output voltage of the OP amplifier 3 is raised. As a result, an increase in the base current of the transistor Q1 is restricted and an increase in the collector current is limited. Consequently, the transistor Q1 can be protected from breakdown (which is a fourth conventional art).
For the secondary output, moreover, there has been proposed a structure in which a resistor R31 is inserted in a current path of a secondary output as shown in FIG. 9A. Alternatively, there has been proposed a structure in which a resistor R32 is inserted in the ground level side to detect a current value of the secondary output as shown in FIG. 9B. Moreover, there has been proposed a structure in which both a resistor R33 and a resistor R34 are inserted as shown in FIG. 9C.
However, the resistors R31 and R33 shown in FIGS. 9A and 9C are elements for controlling a maximum value of a discharge current of a capacitor C4 when a secondary output is short-circuited, there by preventing the capacitor C4 from being deteriorated. Moreover, the resistors R32 and R34 are elements for detecting the current oft the secondary output. Accordingly, the values of the resistors R31 to R34 are set to be comparatively small. As a result, when the secondary output is short-circuited, the load of the secondary coil L2 is too increased so that the oscillation of the transistor Q1 is stopped. When the oscillation is stopped, the collector current of the transistor Q1 is increased even if the third conventional art or the fourth conventional art is used. At this time, it is hard to reduce an increase in the current of the collector. For this reason, when the output is short-circuited, the calorific value of the transistor Q1 is increased. Accordingly, it is necessary to attach a radiator to the transistor Q1 or to set the transistor Q1 to be a large-sized element having a great allowable heat loss in order to prevent the thermal breakdown of the transistor Q1 during the short circuit of the output. Consequently, the cost of parts is increased.
Furthermore, also in the case in which the radiator is attached to the transistor Q1 or the transistor Q1 is set to be a large-sized element having a great allowable heat loss, the following problem arises. More specifically, it is dangerous to increase the length of a path for a high voltage sent from a high voltage generating circuit. Therefore, the high voltage generating circuit is provided in the vicinity of a toner system printing section having a photosensitive drum. Accordingly, when the secondary output is short-circuited so that the calorific value of the high voltage generating circuit is increased, an environmental temperature of the toner system printing section is raised. For this reason, the temperature of rollers of the photosensitive drum is raised so that the performance of the rollers is deteriorated. As a result, it is hard to uniformly charge the surfaces of the rollers. Consequently, printing performance is deteriorated.
The invention has been made to solve the problems and has an object to provide a high voltage generating circuit for a toner system printer which can restrict a lower limit of a load impedance of a secondary coil to prevent the oscillation of a transistor for oscillation from being stopped during a short circuit of a secondary output and can carry out intermittent oscillation to suppress an increase in the heat generation of the transistor for oscillation during the short circuit of the output, thereby preventing the performance of a toner system printing section from being deteriorated due to a rise in a temperature.
Another object of the invention is to provide a high voltage generating circuit for a toner system printer which can suppress an increase in the number of elements also during intermittent oscillation by using a circuit for restricting a lower limit of a load impedance as a circuit for detecting a short circuit of an output.
Still another object of the invention is to provide a high voltage generating circuit for a toner system printer which can suppress an increase in the number of elements also during intermittent oscillation by using a capacitor for changing a PWM signal into a direct current as a capacitor for delaying the restart of oscillation in the intermittent oscillation.
Yet still another object of the invention is to provide a high voltage generating circuit for a toner system printer which can optionally delay a period required for restarting the oscillation in the intermittent oscillation without causing an increase in a time constant of a circuit for changing a PWM signal into a direct current.
Yet still another object of the invention is to provide a high voltage generating circuit for a toner system printer which can cause the oscillation in the intermittent oscillation to sharply rise without an increase in the number of elements.
In order to solve the problem, the present invention provides a high voltage generating circuit for a toner system printer comprising a transformer having at least a primary coil, a secondary coil and an auxiliary coil wound thereon, a transistor for oscillation having a collector connected to the primary coil and a base connected to a first terminal of the auxiliary coil and serving to carry out self-oscillation, and a voltage control circuit for sending, to a second terminal of the auxiliary coil, a voltage control signal generated based on a voltage detection signal indicative of a voltage of a secondary output, wherein the voltage control circuit stabilizes the voltage of the secondary output by controlling an oscillation amplitude of the transistor for oscillation in response to the voltage control signal, and the secondary output is led to a high voltage applying portion of a toner system printing section, the high voltage generating circuit further comprising a load suppressing circuit inserted in a current path of the secondary coil and serving to restrict a lower limit of a load impedance of the secondary coil, thereby preventing the oscillation of the transistor for oscillation from being stopped when the secondary output is short-circuited, and an intermittent control circuit for repeating an operation for reducing a voltage of the voltage control signal to stop the oscillation of the transistor for oscillation when detecting a short circuit of a load, and for raising the voltage of the voltage control signal to restart the oscillation of the transistor for oscillation after stopping the oscillation of the transistor for oscillation.
More specifically, the load suppressing circuit prevents the transistor for oscillation from stopping the oscillation also when the secondary output is short-circuited. Accordingly, also in the case in which the secondary output is short-circuited, the transistor for oscillation carries out the oscillation when the intermittent control circuit is to raise the voltage of the voltage control signal. Moreover, when the intermittent control circuit is to reduce the voltage of the voltage control signal, the transistor for oscillation stops the oscillation. Consequently, also when the secondary output is short-circuited, the transistor for oscillation carries out the intermittent oscillation. Therefore, the heat generation of the transistor for oscillation can be suppressed.
In addition to the structure described above, moreover, the load suppressing circuit is a voltage dividing circuit inserted between a ground level side path of a rectifying and smoothing circuit for generating the secondary output having a negative voltage by rectifying and smoothing an output of the secondary coil and a ground level, and the intermittent control circuit detects the short circuit of the secondary output based on a divided voltage sent from the voltage dividing circuit.
More specifically, the load suppressing circuit also serves as a circuit for detecting the short circuit of the secondary output. Therefore, it is not necessary to separately provide a circuit for detecting the short circuit of the secondary output.
In addition to the structure described above, furthermore, there is provided a filter circuit including a resistor for a filter which has one of terminals connected to a signal source of a PWM signal and a capacitor for a filter which is connected between the other terminal of the resistor for a filter and the ground level and serving to change the PWM signal into a direct current, the voltage control circuit changing a level of the voltage control signal based on a voltage setting signal to be an output of the filter circuit and the voltage detection signal, thereby stabilizing the voltage of the secondary output into a voltage corresponding to the voltage setting signal, wherein the intermittent control circuit is a switch circuit for closing a connection between the output of the filter circuit and the ground level when detecting the short circuit of the secondary output, and a rise in a voltage of the output of the filter circuit which is caused when the connection of the switch circuit is opened is delayed by the capacitor for a filter, thereby delaying the restart of the oscillation in the intermittent oscillation.
More specifically, the capacitor for a filter serves as an element for delaying the restart of intermittence in the intermittent oscillation and an element to be the filter for changing a PWM signal into a direct current.
In addition to the structure described above, moreover, the intermittent control circuit includes a diode having an anode connected to the output of the filter circuit and a cathode connected to the switch circuit, and a capacitor for delay which is connected to the cathode of the diode, wherein the capacitor for delay is discharged when the connection of the switch circuit is closed.
More specifically, when the connection of the switch circuit is changed from a closed state to an open state, the capacitor for delay which is set in a discharge state is connected through the diode in parallel with the output of the filter circuit. Moreover, in the case in which the capacitor for delay is not set in the discharge state, the capacitor for delay is separated from the output of the filter circuit by the diode.
In addition to the structure described above, furthermore, the capacitor for delay is connected between an output point for a divided voltage of the voltage dividing circuit and the cathode of the diode.
More specifically, when the voltage between the terminals of the capacitor for delay is raised so that the oscillation is restarted and the divided voltage is raised, the voltage of the terminal on the diode side in the capacitor for delay is raised. Accordingly, the capacitor for delay is separated from the filter circuit through the diode. Consequently, the rise in the output voltage of the filter circuit is only delayed by the capacitor for a filter. Therefore, when the oscillation is restarted, the output voltage of the filter circuit is rapidly raised.